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García Fernández, Luis Eduardo (2019) Co-combustion of coal and biomass in a fluidised bed reactor. PhD thesis, University of Nottingham.

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Abstract

For decades coal has been the main energy source for electricity generation in many parts of the world. Coal combustion is the most intensive producer of CO2, the main greenhouse gas (GHG) in the atmosphere. The continued dominance of fossil fuels, particularly coal, in the energy mix has been going on for decades due to the slow uptake of low-carbon technologies. Among others, co-combustion of coal and biomass is quite a promising alternative in the short-term to achieve major cuts in GHG and pollutant emissions. Nevertheless, the current inability to address technical factors linked to the characteristics of biomass in terms of ash issues during combustion or modification of ash quality limits the increase of biomass ratio in co-combustion. The pursuit of new resources to substitute coal has prompted consideration of the use of woody materials, herbaceous biomass or waste from the agroindustrial sector.

The aim of this research was to investigate the combustion behaviour of coal and biomass blends using a pilot-scale bubbling fluidised bed reactor. The blends were prepared with a traditionally used biomass fuel (i.e. wood) and selected rich in alkali-metals biomass fuels (i.e. wheat straw and spent coffee grounds (SCG)) to investigate the potential of SCG as a fuel and the effect of ilmenite on agglomeration tendency when used as an additive and alternative bed material. To elucidate the interaction between the fuels in the blends and the effect of ash composition, thermogravimetric and ash fusibility studies were performed looking at both the parent fuels and blends.

Initially, thermogravimetric studies focused on understanding the thermal decomposition of coal blended with by-products (spent coffee ground (SCG), and coffee husk (CH)) of coffee crop. Blends with wood were included for comparison purpose. The blends were prepared by blending coal with biomass at 10, 30 and 50 wt%. The experiments were performed under inert and oxidising conditions to elucidate the thermal behaviour and fuel interactions. The results showed that the inclusion of either spent coffee grounds or coffee husk in coal combustion can enhance the combustion performance. Indeed, the blends with spent coffee grounds were found to be more reactive than those with coffee husk and with similar characteristics to the blends with wood. It was attributed to the existence of synergistic effects. These results offer compelling evidence of the potential of by-products from the coffee crop, especially spent coffee ground for energetic applications.

The study of ash fusibility characteristics investigated the ash fusibility temperatures under oxidising conditions of coal ash blended with ash from spent coffee grounds. Blends with wheat straw ash were included for comparison purpose. Particular attention was paid to the blending ratio effect on ash fusibility temperatures. The results showed that SCG is an interesting fuel in co-processing due to the low ash content and high ash fusibility temperatures when blended with coal. The high ash fusibility temperatures of the blends were attributed to the high content of CaO, MgO, and P2O5 in SCG with negligible content of SiO2 and Cl, complementing the high content of Al2O3 and Fe2O3 in coal. Evidence from this study suggests that the high ash fusion temperatures in the blends of coal with spent coffee grounds can reduce the common operational issues related to biomass ash such as agglomeration. This can also provide confidence for large scale thermochemical conversion systems fuelled by such kind of coal and biomass blends to be able to maintain long-term stable operation.

The final part of this research focused on co-combustion experiments in a pilot-scale BFB aiming to investigate the potential of SCG as a fuel, and ilmenite as an additive and alternative bed material. For comparison purpose blends of coal with wheat straw and wood pellets were also combusted in the same BFB reactor under similar combustion conditions. Similarly, kaolin and bauxite, and silica sand were compared to ilmenite when used as an additive and alternative bed material, respectively. The inclusion of biomass in coal combustion showed marked differences in the temperature profiles, gas emissions, and combustion performance. The results showed that coal and SCG blends can be combusted for energy applications with improved combustion efficiency compared to wheat straw and wood commercial pellets. Nevertheless, high N content in SCG represents a challenge in terms of NOx emissions that should be addressed.

The results from experiments using ilmenite showed improved combustion performance due to reduced values of unburnt carbon in ash and CO emissions when ilmenite was used in comparison with the reference case. It was attributed to the improved oxygen transport capacity of ilmenite. Nevertheless, higher NOx emissions in comparison with the reference case were found when ilmenite was used as alternative bed material. Ilmenite showed to have potential to reduce the risk of agglomeration when used as an additive and alternative bed material. SEM/EDX and XRF analysis of agglomerates and ash showed the predominantly presence of K and Si in the binder region in all cases, with other refractory elements (i.e. Ca, Al, Mg, and Ti) to a lesser extent. This indicates the existence in the binder of K-Ca-silicates, K-Al-silicates, K-Mg-silicates, K-Ca/Mg silicates, and Fe- Ti-bearing compounds with higher melting temperatures

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Liu, Hao Snape, Colin
Keywords:	Co-combustion, Spent Coffee Grounds, Bituminous Coal, Thermogravimetic Analysis, Ash Fusibility Temperatures, Fluidised Bed Combustion, Ilmenite, Agglomeration, Synergistic Effects
Subjects:	Q Science > QD Chemistry
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	58955
Depositing User:	Garcia, Eduardo
Date Deposited:	21 Sep 2023 15:18
Last Modified:	22 Sep 2023 04:30
URI:	https://eprints.nottingham.ac.uk/id/eprint/58955

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